Ribonucleotide reductase catalzyes the rate-limiting step in the de novo synthesis of 2'-deoxyribonucleoside 5'-triphosphates. Since this is a key metabolic step in DNA synthesis and hence, cell replication, ribonucleotide reductase has been a target for the design of antitumor agents. In the proposed studies, ribonucleotide reductase will be purified from Ehrlich tumor cells and the enzymological properties of the enzyme will be studied in detail. Structural features of nucleotide analogs and other compounds which serve as inhibitors of ribonucleotide reductase will be probed. Antibodies to the two non-identical protein subunits will be prepared and used to quantitate the levels of these two subunits under various metabolic conditions. These conditions will involve the study of the effects of specific ribonucleotide reductase inhibitors or specific DNA polymerase inhibitors on the levels and turnover of the two subunits which are required for enzymatic activity. The mechanisms of drug resistance to ribonucleotide reductase inhibitors will be studied in L1210 cells in terms of levels of subunits, altered subunits with different enzyme properties or altered metabolism or activation of the agent (e.g. deoxyadenosine). These studies will provide a better understanding of ribonucleotide reductase in terms of the control of enzyme activity via the enzyme proteins, sites which are vulnerable for drug-targeted chemotherapy, and mechanisms by which drug resistance involving the ribonucleotide reductase step develop.